In future digital communication systems including terminals for both data transmission and speech connections (Integrated Services Digital Network, ISDN) the subscribers will be connected to digital exchanges via digital subscriber lines. Through a bus network or a star network, all the terminals will be in communication with a terminal junction point, which in turn is connected to the exchange.
Different solutions are conceivable for multiple access in a digital subscriber connection. One possibility is polling, for example, where the terminal junction point gives permission to the terminals to send in a given order. The method requires that each terminal has an unambigous individual address, which causes large practical problems however.
According to another solution, the terminal wanting to send data information investigates whether the data channel send direction is idle and the channel is then engaged, while if this is not the case there is a wait of one time interval before a new attempt to send is made. If several terminals start sending simultaneously, this is detected and sending is interrupted by all the terminals. Collision is usually detected by the signals on the transmission medium having an unexpected distortion. This method requires that each terminal has access to signal information from all the other terminals, which is a disadvantage.
The aforementioned solutions are described in the publication "Multiaccess Protocols in Packet Communication Systems", IEEE Trans. on Comm, Vol COM-28 NO 4 April 1980, Fouad A. Tobagi.